Substantial forces are required for extrusion of metals to provide profiled metal shapes, for example profiled rails, which may be of light-weight metal. Various parameters are interrelated, such as the temperature of the ram piston, the temperature of the material, that is, of the billet to be extruded, pressure, degree of transformation during extrusion, extrusion press speed, surface condition, and metallurgical characteristics of the formed extrusion. The interrelationships of these parameters are quite complex, and change of any one parameter influences other parameters which, all, interact and interrelate. Upon deformation of the material of the slug or billet during extrusion, internal deformation and frictional heat will be generated in addition to that which is used to heat the slug or billet to extrusion temperature.
The temperature of the extrusion material, and the speed of extrusion, are critical particularly when extruding light-weight metal portions or shapes since heat fissures, referred to as hot short cracks, may occur. U.S. Pat. No. 4,462,234, Florentino et al, the disclosure of which is hereby incorporated by reference, contains a discussion of problems which arise during extrusion.
If the extruded material exceeds a certain critical temperature, it is difficult and sometimes impossible to obtain a perfect smooth surface of the extruded profiled structure. It appears that this is due to remanent eutectic components which become liquid during extrusion and then result in roughened zones at the surface of the extruded structure. To obtain a smooth surface, therefore, the extrusion speed in the past had been held to a range below that in which rough surface portions were observed and, thus, this problem limited the possible extrusion speed although the extrusion presses as a whole could operate at a higher speed. It was also tried to solve the problem of hot short cracking by cooling the die itself. Providing a die with cooling substantially increases the costs of the die. The intensity of cooling, itself, is limited since the die, upon being cooled, is exposed to the danger of formation of fissures.
It has previously been proposed to reduce the cross-sectional area of the billet as it is being extruded in steps and to cool the extruded material in intermediate steps. Such a system is expensive to apply, requires costly tools and dies, and is not practical for use with single-opening dies and extrusion presses.
Cooling the die itself has been proposed, and various such cooling systems are known. U.S. Pat. No. 3,112,828 discloses a single-opening extrusion press die which is interiorly cooled. It has a die opening in the center thereof which is surrounded by a cooling duct. Cooling is effected only in the die itself, that is, after the billet has been already essentially deformed to the shape it should have. The arrangement requires forming the requisite cooling ducts for the cooling system in each die. This substantially increases the cost of making the dies. Cooling the interior of the die limits the cross-sectional area which can be used and weakens the die which, of course, is highly stressed mechanically. Additional weakening is due to the temperature difference or temperature gradient within the interior of the die.
German Pat. No. 22 40 391 describes a solution similar to that of the U.S. Pat. No. 3,112,828, applied to a multiple-opening die. The cooling duct system is provided both in a pre-shaping die and in a final or finishing die. The system has the disadvantage that the cooling ducts must be formed by machining the die and cutting it into the die and, further, matching the shape and arrangement of the cooling ducts to the respective shapes of the profiles which are to be extruded. The die itself is weakened, and, due to the temperature gradient which results, becomes subject to cracking and formation of fissures. The cooling arrangement, primarily, was designed to control the exit speed of the respective individual shaped structural elements so that from one billet, rods or rails of essentially the same length can be obtained.
German Pat. No. 22 11 645 describes an arrangement in which the exit of the die is cooled at the exit surface. Such an arrangement is suitable primarily when using liquid nitrogen as the cooling medium. Liquid nitrogen will exit in gaseous form which can be directed on the extruded rail or rod. This arrangement, as well as the operation of the cooling system, depends on the shape of the rod to be extruded.
German Pat. No. 429,376 describes an arrangement in which a die extends into a housing defining an extrusion chamber, and the outside of the die is cooled at the portion external to the extrusion chamber.
U.S. Pat. No. 4,462,234, Florentino et al, describes a two-step die which is cooled in a region external of the extrusion chamber. Such a cooling arrangement is suitable, for all practical purposes, only for single-opening dies. The cooling arrangement must be matched to the shape of the die and is dependent thereon.